A magnetic disk device controls a position in a radial direction of a head by a function to prohibit write processing in a case where the head exceeds an allowable value (drift-off level: DOL) of off-track in the radial direction of a disk when writing data to a particular track, and controls a track width of a track (hereinafter referred to as adjacent track) adjacent in the radial direction to the particular track. The allowable value of off-track in the radial direction of the particular track is set according to a highest bit error rate in bit error rates of sectors of the adjacent track of a case where the adjacent track is read immediately after written, for example. Therefore, there is a possibility that achievement of high track per inch (TPI) of the magnetic disk device becomes difficult.